ECHIS'ODERMATA. 211 



which it is transmitted to the parietes of the body, and returns by 

 the cloaca to form the intestinal artery. 



The ampulla Poliana (^fig- 100. a), which is double in some species, 

 is the homologue of the blind sacculi, which supply the circular chyla- 

 queous canal sending off the vessels to the bases of the feet in the 

 Asterias. It transmits its fluid principally to an annular reservoir 

 round the pharynx (6), whence proceed the canals of the oral ten- 

 tacula (c) and those supplying the tubular tentacles or feet which 

 perforate the coriaceous integument. The latter canals (</, d) run 

 down in the interspaces of the pairs of muscles, and distribute trans- 

 verse branches to the bases of the tubes as they proceed. 



Hunter * has figured certain glandular sacs opening into the stem 

 of the hollow branchiae (/i, w), which may be regarded as a rudi- 

 mental form of an excretory or renal system. 



The chief divisions of the nervous system consist of the pharyngeal 

 ring, which is closely applied against the inner side of the circle of 

 calcareous plates, and of the flattened chords which proceed, external 

 to the chylaqueous canals, along the groove or middle interspace in 

 each of the pairs of longitudinal muscles. These muscles are 

 attached anteriorly to the calcareous plates, are arranged in five 

 pairs in most Holothurise, and traverse the interior of the integument 

 of the animal through its entire length. The integument is also 

 acted upon by transverse fibres which run external to the longitu- 

 dinal bands ; and such is the irritability of this muscular system, 

 that when the Holothuria is disturbed or captured it will sometimes 

 eject its sand-laden intestine and most of the other viscera by the 

 cloacal aperture, and very effectually unfit itself for anatomical in- 

 vestigations. The longitudinal muscular bands are more numerous 

 in the Sipunculus, and the stratum of circular fibres is more dis- 

 tinctly developed. The disposition of the nervous system accom- 

 panies this progress towards the annulose type, and only a single 

 ventral chord is sent off" from the pharyngeal collar. f 



The generative organs of the Holothuriae constitute, as in other 

 Echinoderms, a very considerable part of the abdominal viscera in 

 the breeding season ; but they present a more complicated form : 

 they consist of a branched system of long and slender caecal 

 tubes (yfig' 100. r), opening externally by a single common canal, 

 whose orifice is near the mouth. The generative organ of the male 

 Holothuria resembles that of the female in structure ; but the sexes 

 may be readily recognised at the breeding season by the different 

 character of the contents of the tubes, which are white or colourless 

 in the male, whilst the ova present a reddish or yellowish hue. 



* X, VoL L pi. 3. t Prep. No. 1292. 



p 2 



